This invention relates to a laser light source with reduced sensitivity to optical feedback effects.
A laser emits light within a very narrow band of frequencies. For example, certain diode lasers that emit light at a wavelength of about 1300 nm in air, corresponding to a frequency of about 200 THz, have a line width of less than 100 MHz, or one part in about two million. When some portion of this emitted optical energy is returned to the laser, for example by reflection or scattering processes, the intensity and spectral properties of the laser emission are changed. These changes are particularly severe in the case of diode lasers, and typically have adverse effects on the system containing the laser.
As an example, the light emitted by a diode laser may be used to monitor an event by directing the light onto a transducer that reflects light with an intensity that depends on the event. A portion of the reflected light is applied to a detector, such as a photodiode, which generates a signal that depends on the intensity of the reflected light and hence contains information regarding the event. In such an arrangement, it is inevitable that some of the light emitted by the diode laser will be returned to the diode laser. For example, if an optical fiber is used to direct the light from the diode laser to the transducer, light may be reflected from the end faces of the optical fiber. Also, some of the light reflected from the transducer will be returned to the diode laser. In order to be able to extract information regarding the event from the signal generated by the detector, the intensity of the light emitted by the diode laser must be known. However, when light of frequency comparable to that emitted by the laser enters the laser, it may cause a severe perturbation in the operation of the laser, and in particular it may cause substantial variations in the intensity and spectral distribution of the light emitted by the laser. These variations make it difficult to extract useful information from the signal generated by the detector.
As a second example, light emitted by a laser may be used to excite a device under test (DUT) in some fashion, and the nature of the excitation may be dependent on the intensity and/or spectral distribution of the light incident on the DUT. If light emitted by the laser is returned to the laser and causes variations in the intensity and spectral distribution of the light emitted by the laser, this may affect the excitation of the DUT.
When light is propagated through a body of electro-optic material, and an electric field of the proper orientation exists in some portion of the electro-optic material through which the light propagates, the light undergoes a phase shift that depends on the integral with respect to propagation distance of the components of the electric field which interact with the optical field via the electro-optic effect. A particularly sensitive implementation of such an electro-optic phase shifter utilizes an optical waveguide in the body of electro-optic material. The waveguide allows confinement of the optical and electric fields in a small physical volume, thereby maximizing the strength of the interaction. For any particular choice of optical axis of propagation, the magnitude of the phase shift is unchanged by reversing the propagation direction.